A study by light and electron microscopy of the fine structure and innervation of the musculature of trachea, bronchi and pulmonary blood vessels in mammals. Systematic and comparative study based on quantitative and morphometric methods. Study of a large set of structural parameters, ranging from the three-dimensional arrangement of muscle cells, cell layers and cell bundles, to the packing density and size of muscle cells, the amount of extracellular material and the occurrence and distribution of non-muscle cells (such as nerves, endothelial cells, interstitial cells, fibroblasts), and to the cytological features of the muscle cells, including cell-to-cell, cell-to-stroma and nerve-muscle junctions. Comparison between various segments of the airways in normal adult mice, rats, guinea-pigs and rabbits. Comparison of the musculature of airways and pulmonary vessels in rats and rabbits from birth to senescence. Comparison of the musculature and its innervation in laboratory rats (and rabbits), in wild rats (and rabbits) and in germ-free rats. Characterization of the structural changes in the musculature in rats exposed to chronic hypoxia and/or hypercapnia. The aim of these comparisons is to define the specific features of each individual muscle and to help to understand the functional significance of the variety of structural features, as a necessary step to form a basis for the study of muscle pathology.